border-1px($color)
  position:relative
  &:after
    display: block
    position:absolute
    left:0
    bottom:0
    width:100%
    border-top:1px solid $color
    content:' '

border-none()
  &:after
    display:none

//处理2x或3x的图片
bg-img($url)
  background-image:url($url+"@2x.png")
  @media (-webkit-min-device-pixel-ratio: 3),(min-device-pixel-ratio:3)
    background-image:url($url+"@3x.png")